TheStranger

Suspects need to be on the ground to arrest them, after you did that call a Transport.

You are correct. I think it is a translation error or something deprecated.
E & Q are indeed (absurdly) much used keys in LSPD:FR, but duty-toggling they cant.
You can use console
Press F4
then type:
ForceDuty
Console will smart type the cmd for you when it is correct, just press TAB, then ENTER
ForceDuty will toggle you both in and out of duty, depending on your current state, of-cause

Arrests are working, but can be a bit cluncky in base-game.
Arrest is initiated by pointing on a subject that has its hand up!
The subject will then go to the ground, and lay on its stomach -Thats where the voes begin..
I dont know the real procedures for real police-officers that are making a real arrest, but maybe the 'clunkyness' actually is emulation of the real sequence?
At this stage -It is clunky to get control over the subject!
The key is to move in on the subject from LEFT! -in aprox. 45 dg angle.
If you 'aim' at their armpit as you move slowly forward, you have success almost every time.
You will hear a dong, and a menu appears in left corner
Point at the subject where you want the handcuff to be placed, and hold E
That handcufs the subject, and makes it stand up.
Important!!
Do not move! during this sequence!!
When he stands, tab E. You grab him and you can now walk him to a position out of traffic.
At that point again tab E and do not move!
Then tab Q
Suspect will kneel and you can now call back-up-transport.
Unfortunately suspect transport in own car is atm not working, i am fighting that in a program atm : /

Oki, thats base-game.
Most will use Stop-the-ped, (STP) witch is an addon-plugin. Supposedly "safe to use".
The page where it is hosted is ABSOLUTELY N O T save -At ALL
Filthy like... : /